Compounds generally known also as keratin reducing substances, for example, thioglycollic acid, cysteine, acetylcysteine, and salts thereof, have hitherto been used for permanent waving for hair (hereinafter often referred to as “permanent hair waving”). Since these conventional compounds known also as keratin reducing substances have properties satisfactory for practical use under alkaline conditions in permanent hair waving, most permanent wave solutions are adjusted to an alkaline pH value of about 9.5. The permanent wave solution adjusted to the alkaline pH value, however, is known to cause damage to the hair and scalp. In order to overcome this drawback, keratin reducing substances usable in the neutral to weakly acidic pH range (pH: 3 to 7.5, 25° C.) are under development.
For example, monoglycerol esters of thioglycollic acid are disclosed as a thioglycollic acid ester having a hydroxyl group in its alcohol moiety (patent document 1). Thioglycollic acid monoglycerol esters, however, have not been put to practical use, because, for the thioglycollic acid monoglycerol esters, there is a report about sensitization that is considered to be derived from the hydroxyl group in the structure.
Further, the use of mercaptoglycollic acid amide derivatives (patent document 2) and mercaptolactic acid amide derivatives (patent document 3) for solving the skin disorder problem caused by thioglycollic acid esters is disclosed. However, it is known that mercaptocarboxylic acid amides are irritant to the skin. Mercaptocarboxylic acid amide derivatives also have a fear of causing the same sensitization. Further, disadvantageously, there is a fear of causing sensitization, irritation to the skin and the like due to unsatisfactory purification and the starting amine liberated from during storage.
Further, cysteamines claimed to exhibit the effect under weakly acidic condition have been studied (patent document 4). Cysteamines, however, suffer from many problems, for example, unsatisfactory waving properties in a weakly acidic to acidic region and the occurrence of peculiar odor from the hair after permanent waving treatment.
[Patent document 1] Japanese Patent Laid-Open No. 291031/1996
[Patent document 2] Japanese Translations of PCT Publication No. 507272/2000
[Patent document 3] Japanese Translations of PCT Publication No. 528901/2003
[Patent document 4] Japanese Patent Laid-Open No. 271214/1991